The effect of various gases (O-2, hydrocarbons, CO, H-2, NO (x) , SO2, and H2O vapor) presenting in the diesel exhaust on soot combustion using LaCoO3 as a catalytic material was investigated in this paper. A significant promotion of the combustion rate was found following a trend of 10% H2O addition > 3,000 ppm NO (x) > 1% H-2 or 3,000 ppm C3H6 addition, while the improvement in soot oxidation due to the introduction of 3,000 ppm CO or 3,000 ppm CH4 into the reactant gas is relatively less. The wet pretreatment of LaCoO3 with 10% steam before soot oxidation hardly affects the combustion behavior. Interestingly, 10% water vapor in the reaction feed produced a significant promoting effect on combustion. In contrast, 30 ppm SO2 treating led to an obvious deactivation likely owing to the coverage of active sites by sulfate compounds.